Utilization of distributed generator inverters as STATCOM

ABSTRACT

The invention provides a method and system for operating a solar farm inverter as a Flexible AC Transmission System (FACTS) device—a STATCOM—for voltage control. The solar farm inverter can provide voltage regulation, damping enhancement, stability improvement and other benefits provided by FACTS devices. In one embodiment, the solar farm operating as a STATCOM at night is employed to increase the connectivity of neighboring wind farms that produce peak power at night due to high winds, but are unable to connect due to voltage regulation issues. The present invention can also operate during the day because there remains inverter capacity after real power export by the solar farm. Additional auxiliary controllers are incorporated in the solar farm inverter to enhance damping and stability, and provide other benefits provided by FACTS devices.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/391,699 filed May 7, 2012, which is a US National Stage (371) ofPCT/CA2010/001419 filed Sep. 15, 2010, which claims the benefit of U.S.Provisional Application Nos. 61/242,501 and 61/309,612 filed on Sep. 15,2009 and Mar. 2, 2010, respectively.

TECHNICAL FIELD

The present invention relates to distributed power generation systems.More particularly, the present invention relates to the use of solarfarm inverters and wind turbine generator inverters as FlexibleAlternating Current (AC) Transmission Systems (FACTS) controller—staticsynchronous compensator (STATCOM).

BACKGROUND

Due to ever-increasing energy demands, depletion of fossil fuel, andenvironmental constraints, the interest in generating green energy atall levels is at an all time peak. Worldwide, governmental incentivesand subsidy programs are attracting several customers to install smallcapacity (ranging from few watts to few kW) renewable energy modules intheir premises. Similarly, large companies are building PV solar farmsranging from few hundred kW to few MW or higher capacity. Distributedgeneration (DG)—power sources connected at one or more locations withinthe distribution system have brought new issues and problems to theexisting power system.

The penetration level of DG systems, such as renewable-energy based DGsystems, is growing. As such, the utility companies are facing majorchallenges of grid-integrating these increasing number sources of power.Challenges such as ensuring voltage regulation, system stability andpower quality within standard limits, are at the forefront of theseproblems.

FACTS devices offer a viable solution to this problem and are beingincreasingly employed in power systems worldwide. FACTS are defined hereas alternating current transmission systems incorporatingpower-electronic based and other static controllers to enhancecontrollability and increase power transfer capability. FACTS devicesare typically utilized for accomplishing the following objectives:

-   -   Voltage control    -   Increase/control of power transmission capacity in a line, and        for preventing loop flows    -   Improvement of system transient stability limit    -   Enhancement of system damping    -   Mitigation of sub-synchronous resonance    -   Alleviation of voltage instability    -   Limiting short circuit currents    -   Improvement of HVDC converter terminal performance    -   Grid Integration of Wind Power Generation Systems

Some of the devices/controllers in the family of the FACTS device thathave been used for achieving any or all of the above objectives areStatic Var Compensators (SVC) and Static Synchronous Compensators(STATCOM), etc.

A static synchronous compensator (STATCOM) is a shunt connected reactivepower compensation device that is capable of generating and/or absorbingreactive power whose output can be varied to control specific parametersof an electrical power system. In general terms, a STATCOM is asolid-state switching converter that is capable of independentlygenerating or absorbing controllable real and reactive power at itsoutput terminals when it is fed from an energy source or an energystorage device at its input terminals.

More specifically, the STATCOM is a voltage source converter thatproduces from a given input of direct current (DC) voltage a set ofthree-phase AC output voltages. Each output voltage is in phase with andis coupled to the corresponding AC system voltage through a relativelysmall reactance (which can be provided either by an interface reactor orleakage inductance of a coupling transformer). The DC voltage isprovided by an energy storage capacitor.

It is also known in the prior art that a STATCOM provides desiredreactive power generation, as well as power absorption, by means ofelectronic processing of voltage and current waveforms in a voltagesource converter (VSC). The STATCOM also provides voltage support bygenerating or absorbing reactive power at the point of common coupling(PCC) without the need for large external reactors or capacitor banks.Therefore, the STATCOM occupies a much smaller physical footprint.

For purposes of this document, a converter is a general name for bothrectifiers and inverters.

It also known that a STATCOM can improve power system performance inareas such as:

-   -   Voltage control    -   Increase/control of power transmission capacity in a line, and        for preventing loop flows    -   Improvement of system transient stability limit    -   Enhancement of system damping    -   Mitigation of sub-synchronous resonance    -   Alleviation of voltage instability    -   Limiting short circuit currents    -   Improvement of HVDC converter terminal performance    -   Grid Integration of Wind Power Generation Systems    -   Voltage flicker control; and    -   Control of reactive power and also, if needed, active power in        the connected line (this configuration requires a DC energy        source).

The reactive and real power exchange between the STATCOM and the ACsystem can be controlled independently of one other. Any combination ofthe real power generation/absorption together with reactive powergeneration/absorption is achievable, if the STATCOM is equipped with anenergy storage device of suitable capacity. With this capability, someextremely effective control strategies for the modulation of thereactive and real output power can be devised to improve the transientand dynamic system stability limits.

The increasing penetration level of DG systems in modern powertransmission and distribution systems is presenting several technicalchallenges. One of these challenges is the voltage variation along thefeeder. Traditionally, the direction of electrical power flow has beenfrom the grid towards the loads connected in the distribution feeders.The voltage drop over the feeder length was tackled effectively byadjusting the sending end voltage magnitude or by providing reactivepower support at one or more locations in the transmission/distributionfeeders. To maintain the voltage at different locations within thestandard limits, the utility companies traditionally use a combinationof on-line tap changing transformers, and capacitor banks at differentlocations.

A DG system dominated by wind farms, however, may exhibit an interestingcondition, especially at night. At this time, the electrical loads aremuch lower than their day-time values, given that the wind turbinegenerator outputs are much higher due to high wind speeds in the nightcompared to day. This increased power generated from wind farms at nightcan cause significant amount of power to flow in the reverse directiontowards the main grid. Since the present power distribution systems weredesigned and operated with an important assumption of power alwaysflowing from main grid towards the end users, this reverse power flowcondition causes the feeder voltages to rise above their normal ratedvalues. In certain cases, this increase in voltage can exceed thetypically allowable limit of ±5%. This is not acceptable to electricutilities.

The problem of reverse power flow presents a major challenge when addingmore DG systems to a feeder line. Maintaining the voltage rise withinthe specified range directly affects the number of DG systems that canbe connected on a particular distribution network. When addingadditional wind farms to a network, utilities may be forced to installexpensive voltage regulating devices in the family of FACTS controllers,such as an SVC or a STATCOM to mitigate this problem.

In light of the above, there is a need for a system, method, and/ordevice for adapting existing DG systems to support the addition of windfarms and other DG sources without requiring expensive voltageregulation devices.

SUMMARY

The present invention provides a solution to this problem by utilizingPV solar farms as not only a source of real power but also a source ofdynamically controllable reactive power.

In particular, the invention provides a method of operating a solar farminverter primarily as a STATCOM during the night to mitigate the highvoltages caused by the addition of wind farms to a DG system. Thepresent invention demonstrates that a solar farm inverter can beeffectively utilized to regulate the voltage at point of common coupling(PCC)—the location where the wind farm is integrated. Furthermore, atnight time, the solar farm can be utilized to achieve all the possiblefunctions of a STATCOM for improving the power system performance byincreasing system stability, damping power system oscillations,alleviating voltage instability, suppressing subsynchronous resonance,etc. It can also be utilized to provide load reactive powersupport/compensation, perform load balancing, and/or neutralize loadcurrent harmonics.

The entire rating of the solar farm inverter is available foraccomplishing the above functions, since the solar farm is absolutelyidle and not producing real power at night times as the sun is absent.During the day-time when power generation from the solar farm is not ata peak (such as during early morning and late afternoon hours), theremaining solar farm inverter capacity can be utilized to perform any orall of the above mentioned tasks/functions.

In a further embodiment, the present invention further provides anauxiliary controller having a plurality of modes of operation. Thecontroller is capable of performing voltage regulation, during thenight-time and day-time operation of the DG systems.

In addition, the present invention includes a further embodiment ofproviding a voltage controller and an auxiliary damping controller. Thevoltage controller and the damping controller operate with the inverterbased solar DG connected to the grid or the inverter based wind DGconnected to the grid. This further embodiment improves the transientstability of the DG system both in the night and the day time wheneverthere is an availability of reactive power capacity in the DG system.

In a first aspect, the present invention provides a distributed powergeneration source, for operatively connecting to a distributed powergeneration network at a point of common coupling, said distributed powergeneration source comprising:

-   -   a voltage inverter    -   a control means that operates said voltage inverter, wherein a        voltage at said point of common coupling is regulated by the        control means, as a static synchronous compensator (STATCOM),        when said distributed power generation source is providing less        than its maximum rated active power to said network,        wherein said STATCOM prevents said voltage at said point of        common coupling from exceeding a voltage rating when at least        one more additional distributed power generation sources being        operatively connected to said network produces an excess amount        of power relative to an amount of power required by one or more        loads on said network.

In a second aspect, the present invention provides a control system foruse in controlling a distributed generation (DG) power source havingmultiple functions relating to a power transmission system, the systemcomprising:

-   -   a master control unit producing a digital control word, said        control word having distinct sections; and    -   a plurality of control modules, each control module being for        producing values for use in a different function of said DG        power source, said values being used to produce a signal        proportional to a signal required by said different function;    -   wherein each control module receives at least a portion of said        digital control word; and        wherein each control module is activated and deactivated by a        specific distinct section of said digital control word.

In a third aspect, the present invention provides a system for improvingtransient stability of a power transmission system line, the systemcomprising:

-   -   a power source having an output being injected on to said        transmission system;    -   a damping controller receiving as input a signal indicative of        oscillations in of said power transmission system and outputting        a damping control signal    -   a control system receiving said damping control signal;    -   wherein said control system outputs a magnitude control signal        proportional to a transient signal on said transmission system        line; and    -   wherein said magnitude control signal controls said power source        such that said output is based on said magnitude control signal.

In a fourth aspect, the present invention provides a method foroperating an energy farm, said energy farm being connected to a powertransmission system, said energy farm being equipped with an inverter,the method comprising:

-   -   operating said energy farm as a static synchronous compensator        (STATCOM) using said inverter    -   increasing the transmission capacity of said transmission system        using said inverter    -   charging users of said transmission system for an increase in        said transmission capacity of said transmission system.

In a fifth aspect, the present invention provides a method for operatinga solar energy farm, said solar energy farm being connected to a powertransmission system shared with at least one other energy farm, saidenergy farm being equipped with an inverter, the method comprising:

-   -   coupling said solar energy farm to at least one other energy        farm    -   operating said solar energy farm as a static synchronous        compensator (STATCOM) using said inverter    -   controlling a voltage on said power transmission system using        said inverter    -   charging users of said transmission system for using said        inverter as a voltage regulating device on said transmission        system.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention will now be described byreference to the following figures, in which identical referencenumerals in different figures indicate identical elements and in which:

FIG. 1 shows a system block diagram representation of an embodiment ofthe present invention.

FIG. 2 shows a detailed representation of a PV (PV) solar farm.

FIG. 3 shows a simplified system configuration of an embodiment of thepresent invention.

FIG. 4 shows a phasor representation of voltage drop compensationutilizing the PV solar farm inverter: (a) Night-time operation and (b)Day-time operation.

FIG. 5 shows a phasor representation of voltage rise compensationutilizing the PV solar farm inverter: (a) Night-time operation and (b)Day-time operation.

FIG. 6 shows the present utilization of a PV solar farm over 24hours—(a) Day-time operation: PSF<PL, (b) Day-time operation: PSF=PL,(c) Day-time operation: PSF>PL, and (d) Night-time operation: PSF=0.

FIG. 7 shows different modes of operation of a PV solar farm duringnight-time according to an embodiment of the present invention.

FIG. 8 shows additional modes of operation of a PV solar farm duringnight-time.

FIG. 9 shows a PV solar farm inverter active-reactive powers (P-Q)capability curve.

FIG. 10 shows a block diagram representation of control scheme used toimplement an embodiment of the present invention.

FIG. 11 shows a block diagram representation for hysteresis currentcontrol operation.

FIG. 12 shows a flow chart to activate particular mode of operation.

FIG. 13 shows a line diagram of (a) study system I with single solarfarm and (b) study System II with a solar and a wind farm according toan embodiment of the present invention.

FIGS. 14 (a), (b), and (c) shows block diagrams of the varioussubsystems in the two equivalent DGs in accordance with a furtherembodiment of the present invention.

The Figures are not to scale and some features may be exaggerated orminimized to show details of particular elements while related elementsmay have been eliminated to prevent obscuring novel aspects. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting but merely as a basis for the claims and as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

DETAILED DESCRIPTION

Generally speaking, the systems described herein are directed to amethod of regulating the voltage in a DG system using a solar farminverter as a STATCOM, especially during night time. As required,embodiments of the present invention are disclosed herein. However, thedisclosed embodiments are merely exemplary, and it should be understoodthat the invention may be embodied in many various and alternativeforms. For purposes of teaching and not limitation, the illustratedembodiments are directed to a method of regulating the voltage in a DGsystem using a solar farm inverter as a STATCOM.

As used herein, the terms, “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in this specification including claims, theterms, “comprises” and “comprising” and variations thereof mean thespecified features, steps or components are included. These terms arenot to be interpreted to exclude the presence of other features, stepsor components.

The present invention allows solar farm inverters to be controlled as aSTATCOM in the night when there is no sunlight. When used as a STATCOMat night, the entire rating/capacity of solar farm inverter is employedto provide several benefits to the power system as normally provided bythe FACTS technology. During daytime (especially during partial sun,i.e., in early mornings and late afternoons) all the capacity of thesolar farm inverter remaining after that required for real powergeneration is utilized to be controlled as STATCOM. Such an approachallows for a new set of applications and potential revenue earningmethods for solar farms other than simply producing real power duringthe day.

The present invention also allows wind turbine generator inverters(especially for wind turbine generators based on inverter technology) tobe controlled as STATCOM during hours when there is no wind. When windis absent, the entire rating/capacity of the wind turbine inverters areemployed to provide several benefits to the power system as normallyprovided by the FACTS technology. During other times (especially duringless wind regime), all the capacity of the wind turbine invertersremaining after that required for real power generation, is utilized tobe controlled as STATCOM. This opens up a new set of applications andpotential revenue earning to the wind farms than simply from producingreal power.

While the potential applications of PV solar farm as STATCOM (FACTSdevice) are several, the following description illustrates two majorbenefits of solar farm utilization as STATCOM: 1) integrating more windpower systems in the transmission/distribution networks by providingvoltage control on the network, and ii) increasing the stable powertransfer limit on transmission systems through both voltage control andauxiliary damping control.

While the potential applications of wind farm as STATCOM (FACTS device)utilizing auxiliary controls are several, the following descriptionshows one major benefit of wind farm utilization as STATCOM: increasingthe stable power transfer limit on transmission systems through bothvoltage control and auxiliary damping control.

The utilization of solar farm inverters and wind farm inverters asSTATCOM is applicable regardless of the following: 1) type andconfiguration of inverter e.g., 6 pulse, 12 pulse, multilevel, etc, 2)type of semiconductor switches used is inverters, e.g. GTO, IGBT, etc,3) type of firing methodology used, PWM, SPWM, hysteresis control, PLLbased, etc., 4) methodology of controller design, e.g., pole placement,lead lag control, genetic algorithm based control, etc, 5) choice ofauxiliary control signals, e.g., local signals such as line currentmagnitude, active power flow, local bus frequency, remote signals suchas phasor measurement unit (PMU) acquired signals, etc.

Table 1 below provides an explanation for the various terms and notationused in different figures and in the description below.

Symbol Description v_(PCC,a) = v_(PCC,a) Instantaneous phase-a voltageat PCC (ωt) v_(PCC,b) = v_(PCC,b) Instantaneous phase-b voltage at PCC(ωt) v_(PCC,c) = v_(PCC,c) Instantaneous phase-c voltage at PCC (ωt)V_(m) Peak magnitude of rated voltage at PCC V_(PCC) Peak value ofactual voltage at PCC V*_(PCC) Peak value of reference (desired) voltageat PCC V_(dc) Actual DC bus voltage V*_(dc) Reference (desired) DC busvoltage I_(v) Required magnitude of current to achieve PCC voltagecontrol I_(DC) Required magnitude of current to achieve DC bus voltagecontrol i_(va) = i_(va) (ωt) Instantaneous phase-a reference current forPCC voltage control i_(vb) = i_(vb) (ωt) Instantaneous phase-b referencecurrent for PCC voltage control i_(vc) = i_(vc) (ωt) Instantaneousphase-c reference current for PCC voltage control i_(dc,a) = i_(dc,a)(ωt) Instantaneous phase-a reference current for DC bus voltage controli_(dc,b) = i_(dc,b) (ωt) Instantaneous phase-b reference current for DCbus voltage control i_(dc,c) = i_(dc,c) (ωt) Instantaneous phase-creference current for DC bus voltage control i*_(SF,a) = i*_(SF,a) Netinstantaneous phase-a reference current for SF- (ωt) inverter controli*_(SF,b) = i*_(SF,b) Net instantaneous phase-b reference current forSF- (ωt) inverter control i*_(SF,c) = i*_(SF,c) Net instantaneousphase-c reference current for SF- (ωt) inverter control U_(a) Phase-aPCC voltage in per unit (pu) form U_(b) Phase-b PCC voltage in pu formU_(c) Phase-c PCC voltage in pu form k Voltage gain to convert actualPCC voltages to pu value k_(v) Voltage gain to convert pu value toactual value k_(DC) Voltage gain to convert pu value to actual value CdcDC link capacitor Lsh Interfacing series inductor S1 to S6 InsulatedGate Bipolar Transistors (IGBTs) G1 to G6 Gate switching pulses to turnON/ OFF the IGBTs Capital Letters Peak/Average/DC or Root mean-square(rms) values (Ex. V_(PCC); V_(dc)) Small Letters Instantaneous valueswhich vary with time (Ex. v_(PCC,a); i*_(SF,a))

The present invention provides a method for utilizing a solar farminverter as a source of both real and reactive power to support thegrowth of DG systems. The invention makes use of the fact that the solarfarm inverter is unutilized during night-time. Additionally, when thesolar farm is not producing power up to its rated generation capacity,the present invention can also be applied during the day-time. Forapproximately 60% of the day-time (8 hours out of 13 hours of daylight),the solar farm inverter capacity is remains underutilized (i.e. invertercapacity is utilized below 75% of its rated capacity). Thisunderutilized inverter capacity can therefore be gainfully employed toachieve the similar functionality as of night-time at, however, alimited scale. For ease of understanding hereafter, the operating modesof the present invention are addressed as night-time mode of operation(or simply “night-time”) and day-time mode of operation (or simply“day-time”).

The present document refers to a photovoltaic (PV) solar farm. However,the skilled artisan will understand that the present invention is notlimited to this type of solar system, but can be used with anydistributed power generation source having a voltage inverter may beutilized.

The spare available solar PV inverter capacity thus can be utilized tosolve several known problems in DG systems. The present inventionprovides several embodiments in which maximum benefits from the solarfarm inverter can be realized. Table 2 highlights the proposedapplications of the solar farm during both modes of operation.Furthermore, some of these applications can be integrated to achievemultiple tasks simultaneously.

TABLE 2 Some Modes of Operation of a Solar Farm Modes Of Operation I.Night-Time Operation II. Day-Time Operation PCC Voltage RegulationActive Power Injection Auxiliary/Damping control PCC Voltage RegulationLoad Reactive Power Compensation Auxiliary/Damping control Power QualityEnhancement Load Reactive Power Compensation Load and/or NetworkBalancing Power Quality Enhancement Battery Charging Load and/or NetworkBalancing

FIG. 1 illustrates the single-line representative diagram of theexemplary system. This system is comprised of a wind farm and a PV solarfarm. The distances between different points of interest are representedby equivalent line impedances, such as, Zl1, Zl2, etc. For simplicity,the loads on the system are combined together, considered at the end ofthe feeder and represented by equivalent MW and MVar.

FIG. 2 is a detailed PV solar farm schematic, modeled as a voltagesource inverter with a DC bus capacitor. The voltage source inverter isrealized by utilizing six semiconductor switches (here, Insulated GateBipolar Transistors (IGBTs)). It may be understood that there areseveral types/configurations of voltage source converters/inverters.However, the invention applies to any type/configuration of theinverter. The inverter is connected to the network through interfacingseries inductors and a step-up transformer. The point at which the PVsolar farm is connected to the feeder/network is termed as point ofcommon coupling (PCC). The currents injected/delivered by the PV solarfarm are denoted as i_(SF,a), i_(SF,b) and i_(SF,c).

As mentioned earlier, the present invention seeks to increase the realpower injection capability of the wind farm, especially during thenight-time when wind farms generally produce more power than in theday-time. When the power generated by the wind farm is greater than theloads connected downstream of the wind farm, the remaining excess powerflows towards the main grid. This reverse power flow causes the feedervoltage to rise. If the amount of the reverse power flow issignificantly high, the feeder voltage level may increase beyond theaccepted limit imposed by the utility (such as ±5% of the rated feedervoltage). If such an event occurs (i.e., feeder voltage more than 1.05per unit due to reverse power flow), the wind farm has to shut down orits output power injection needs to be reduced.

Accordingly, the present invention uses the unutilized PV solar farminverter (during night-time) to control the feeder voltage during suchan event. The PV solar inverter controls and thus restores the increasedfeeder voltage back to the acceptable limit by injecting the appropriateamount of controlled reactive power.

Generally, a capacitor is connected on the DC side of the solarinverter. In the present invention, the voltage across this capacitor(referred to hereafter as the DC link voltage/DC bus voltage) ismaintained at a reference value by taking a small amount of active powerfrom the grid. Inclusion of a self-supporting DC bus feature in a PVsolar farm, especially during the night-time, is required. This enablesthe PV solar farm to perform as a STATCOM.

This section describes the operating principle of using a PV solar farmto regulate the PCC voltage.

The system under consideration as given in FIG. 1 is represented in FIG.3 as a simplified diagram to aid in a better understanding of theoperating principle of the present invention. Furthermore, forsimplicity, the following assumptions are made:

-   -   line resistance and capacitance are neglected;    -   load is connected very close to the solar farm, i.e. zero        impedance between the PV solar farm and the load; and    -   a unity power factor load.

The second assumption of connecting the load very close to the PV solarfarm helps to simplify the phasor diagram as the load and the PCCvoltages will be identical. However, for the more complexrepresentation, the line impedance between the PCC and the load shouldbe included. Under such a condition, the load voltage phasor will havelower/higher magnitude and a phase shift compared to the PCC voltagethat would depend on the length of line Zl2 and the amount of currentdrawn by the load.

In principle, when there is a drop or rise in voltage from its ratedvalue, an externally installed FACTS device, such as a STATCOM, shouldinject appropriate reactive power to counterbalance the voltagedrop/rise across the line impedance and thus restore the voltage closeto the rated value.

When the PV solar farm (acting as STATCOM) injects reactive power(capacitive mode operation), the PCC voltage increases. However, if thePV solar farm acting as STATCOM absorbs reactive power (inductive modeoperation) the PCC Voltage decreases. Hence a controlled reactive powerinjection/absorption in response to the measured value of the PCCvoltage will regulate the PCC voltage and maintain it constant at adesired preset value.

FIG. 4 shows the phasor representation when the PV solar farm inverteris operated and controlled as a STATCOM to compensate for the drop inthe voltage. The voltage at the distribution level (after the step-downtransformer), V_(S), is considered to be a reference phasor. Theeffective voltage drop that is responsible for regulating the PCCvoltage is termed a compensating voltage (V_(C)). The flow of loadcurrent through the feeder causes the voltage to drop across the lineimpedances. For an uncompensated line, as the length of the lineincreases, the effective voltage available at the farthest end graduallydrops. The line impedance is also responsible for the phase angle lagbetween the distribution transformer's secondary and PCC voltages,denoted as δ.

In order to compensate for the drop in voltage at the PCC, the solarfarm is controlled as a capacitor. Figure (a) shows the phasorrepresentation for the PV solar farm inverter compensating for thevoltage drop during night-time. V_(PCC) and V^(*) _(PCC) represent thereduced and reference PCC voltages, respectively. Since the lineresistance is neglected, the quadrature leading current (I_(SFq)), whenflowing through the inductive line impedance, will cause an additivevoltage drop V_(C). This action will boost the reduced PCC voltageV_(PCC) to V*_(PCC). The resultant source current (I′_(S)) is the vectorsum of I_(L) and I_(SFq). The effective phase angle between thetransformer secondary voltage V_(S) and the resultant source currentI′_(S) is denoted as φ′_(S). The phase angle between the voltage acrossthe solar farm (PCC) and its injected current is denoted as φ_(SF).During night-time, phase angle φ_(SF) will be close to 90°.

The compensating voltage V_(C) is a function of the line impedance (Zl)and the quadrature current I_(SFq), which can be expressedmathematically as:|V _(C) |=I _(SFq) ·Z _(l)  (1)

From FIG. 4 (a), V_(C) can also be represented as:|V _(C) |=|V ^(*) _(PCC) |−|V _(PCC)|  (2)

In equation (2), V^(*) _(PCC) is a known quantity and, V_(PCC) (actualPCC voltage) can be measured easily using a voltage sensor. Thus, theamount of the PV solar farm inverter current needed to compensate forthe desired drop in voltage can be calculated as:

$\begin{matrix}{I_{SFq} = \frac{{V_{PCC}^{*}} - {V_{PCC}}}{Z_{l}}} & (3)\end{matrix}$

FIG. 4 (b) shows the phasor representation of voltage drop compensationduring day-time. The compensation principle and all the equations areidentical to those for night-time operation. The only difference is thatthe solar farm inverter provides the reactive power (quadrature current)necessary to achieve the desired voltage boost while delivering the PVgenerated active power to the grid. Therefore, during day-time, the netcurrent injected by the solar farm inverter (I_(SF)) will be the vectorsum of the active (I_(SFa)) and the reactive (I_(SFq)) currentcomponents.

In a preferred embodiment of the invention, the increase in voltage canbe due to the reverse power flow from another DG source on the samefeeder or on a neighbouring feeder or from the solar farm itself(possibly during day-time).

In the nighttime, the entire solar farm inverter capacity is availablefor providing controllable reactive power for voltage regulation.

During the daytime, the inverter capacity remaining after real powerinjection is utilized for providing controllable reactive power forvoltage regulation. For instance, it is only around noon time that thetotal inverter capacity is utilized for real power production. Duringmorning hours and later afternoon hours, only a partial invertercapacity is used up.

FIG. 5 (a) shows the phasor representation of a PV solar farm invertercompensating for the voltage rise during night-time. In order tocompensate the increased voltage at PCC, the solar farm is controlled asan inductor. The lagging current supplied by the solar farm inverter(I_(SFq)) will cause a subtractive voltage drop V_(C) across the lineinductance. The result of this will bring back the excess over voltagewithin the acceptable voltage limit. In FIG. 5 (b) the voltage risecompensation during day-time is shown. Here, the solar farm inverterinjects active and reactive current components simultaneously to achieveovervoltage compensation while injecting active power to the grid.Equations (1) to (3) are also applicable for voltage rise compensation.

It is important to note that the above formulation is based on theassumption of an inductive line (Rl=0). For a more preciserepresentation and calculation, the line resistance should also beconsidered. With a combined inductive and resistive line, when the solarfarm inverter is utilized for voltage regulation, the drop across theresistive element will increase or decrease the phase angle shiftbetween the resultant PCC and distribution transformer secondaryvoltages.

Thus, in a preferred embodiment, the solar farm inverter is operated(both during night-time and day-time) as a FACTS Device—STATCOM toregulate the feeder voltage and to support the expansion of the capacityof a distribution network. The increased capacity enables the additionof distributed power sources that would otherwise cause the line voltageto exceed rated limits at night. In a preferred embodiment, theadditional distributed power sources include one or more wind farmsconnected on the same feeder.

In a preferred embodiment of the invention, the solar farm inverter iscontrolled to perform several other tasks. All these features arerepresented by block diagrams to depict the role of PV solar farm insupporting/injecting the reactive and active powers.

FIG. 6 shows the block diagram representation of a current utilizationof a PV solar farm over a period of 24 hours. The load is assumed to bea combination of active and reactive power loads and the DG system isrepresented only by the solar farm. For better understanding, the flowof powers (active & reactive) at different locations is also highlightedin block diagrams.

FIGS. 6 (a)-(c) represent a typical day-time operation of a PV solarfarm. Under these conditions, the solar farm injects active powergenerated by PV cells and this is termed as the ‘active power injection(API)’ mode of operation. Three possibilities for power generation fromthe solar farm are: (i) power generated by the solar farm (P_(SF)) isless than the load active demand (P_(L)) [FIG. 6 (a)], (ii) P_(SF) isexactly equal to P_(L) [FIG. 6 (b)], and (iii) P_(SF) is greater thanP_(L) [FIG. 6 (c)]. The condition in FIG. 6 (c) represents the reversepower flow.

FIG. 6 (d) shows the block diagram representation of the solar farmduring night-time. Note that the solar farm is inactive during theentire night-time period. In all of the above mentioned operatingscenarios, the reactive power demanded by the load is supplied by thegrid.

The control aspects of the preferred embodiment of the invention aresummarized in FIG. 7 and are briefly addressed hereafter. FIG. 7 (a)depicts the previously discussed invention of the PV solar farm inverteras a STATCOM to regulate the PCC voltage. This mode of operation isreferred to as ‘voltage regulation (VR)’. The reactive power flow Q_(S)during the voltage regulation mode of operation, seen from thedistribution transformer side, will be the vector sum of Q_(L) (if any)and Q_(VR).

Furthermore, in a preferred embodiment of the invention, the PV solarfarm inverter is controlled to damp any power oscillations caused byelectromechanical oscillations (0.8-2 Hz) of synchronous generators inthe grid as well as by any inter-area oscillations (0.1-0.8 Hz) that mayget excited after any disturbance in the power system. It should benoted that these disturbances might come from line/transformer switchingor faults. The solar farm inverter can also be operated to improve thestability limit of the power system thus enabling higher power flows inthe transmission lines in a secure manner. All these control aspects areaccomplished through the auxiliary controller, referred to hereafter asthe Aux. Ctrl.

According to the present invention, the auxiliary controller can bebased on either locally measured signals known as “local” signals, orremotely transmitted signals known as “remote” signals. A property ofthese auxiliary signals is that they contain/reflect the power systemoscillations which need to be damped by the solar farm inverter actingas a STATCOM. Examples of “local signals” are the line active powerflow, the magnitude of line current, the local bus frequency, etc. Onthe other hand, examples of remote signals include remote bus voltages,oscillations of remote generators, and remote line flows, etc. Theseremote signals are made available to the Solar Farm acting as a STATCOMthrough Phasor Measurement Units (PMU) based on GPS technology, or aretransmitted through dedicated fibre optic cables.

The auxiliary controller may utilize a washout filter, a gain element,and a few stages of lead-lag controllers. The output of the auxiliarycontroller adds to the voltage controller. While the voltage controlmode attempts to keep the PCC voltage constant with a very small timeconstant (15-45 msec), the auxiliary damping control allows a smallmodulation of the PCC voltage around the nominal values (with a slowtime constant (0.1-2 sec)). This imparts a damping capability to thesystem when oscillations exist on the network. In absence ofoscillations, only the voltage controller is active.

According to the present invention, if the load on the network demandslagging or leading reactive power, the PV solar farm inverter iscontrolled to support a leading (capacitive) or a lagging (inductive)reactive power. FIGS. 7 (a) and (b) show the flow of reactive power fora lagging power factor and for a leading power factor load condition,respectively. This “load reactive power compensation” (LRPC) mode ofoperation can thus ensure a unity power factor operation at PCC and canalso help to reduce the line losses by an appreciable extent.

The difference between voltage regulation and load reactive powercompensation modes of operation is explained here. When the solar farminverter is used to support lagging or leading load reactive powerdemand, the voltage at PCC is indirectly raised or lowered,respectively, by a certain percentage. This percentage wholly depends onthe amount of reactive power (lagging or leading) required by the load.However, there is no direct control over such voltage regulation. On theother hand, during the voltage regulation mode of operation, improvementin the power factor can also be accomplished. The two issues of voltagecontrol and load power factor correction can be optimally controlled byintegrating these aspects as depicted in FIG. 7 (e).

In a preferred embodiment of the invention, the PV solar farm inverteris also utilized to compensate/neutralize the harmonics generated by anon-linear load and thus can help to reduce the harmonics pollution onthe distribution network. This control feature is referred to as‘harmonic compensation (HC)’ mode of operation. FIG. 7 (f) depicts theinjection of harmonic active and harmonic reactive powers by the PVsolar farm inverter to compensate for the harmonics generated by thenon-linear loads connected downstream of the solar farm.

In the preceding discussion of the embodiments of the invention, thepossible control approaches for the solar farm inverter to achieveindividual functions at the distribution level have been presented.However, on a typical distribution network, a combination of thesefunctions may need to be accomplished. In another preferred embodimentof the invention, the above discussed functions are coordinatedsimultaneously.

These coordinated features are depicted in FIGS. 7 (g), (h) and (i) forthe combined VR/Aux. Ctrl. and HC; LRPC and HC; and VR/Aux. Ctrl. andLRPC and HC compensations, respectively. For a 3-phase 4-wire system,the solar farm inverter can also be utilized to compensate unbalancedload currents drawn by the combination of three-phase and single-phaseloads. The block diagram representation for this feature is not shown inthe FIG. 7.

In another preferred embodiment of the invention, the PV solar farminverter is operated as a fully controlled battery charger especiallyduring the night-time. In this case, the PV solar farm inverter in acombined solar farm and wind farm DG system is utilized in conjunctionwith energy storage batteries to store the excessive power generated bythe wind farm. This feature performs two functions: (i) improving thesystem reliability by releasing the stored battery charge during peakload condition and, (ii) the real power storage during the chargingprocess helps to regulate the rise in feeder voltage if controlled in anappropriate manner.

The solar farm inverter during the day-time should necessarily injectactive power generated by the PV solar cells. While injecting the activepower to the grid, the solar farm inverter can be additionallycontrolled to achieve the features discussed earlier in this document.However, the available solar farm inverter rating may impose alimitation on the amount of reactive power that can be injected duringthe day-time.

For a comprehensive overview, four block diagram representations of aproposed day-time operation are shown in FIG. 8. The block diagramrepresentation for combined API & VR/Aux. Ctrl., API & LRPC and API & HCcompensations are shown in FIGS. 8 (a), (b) and (c), respectively. FIG.8 (d) shows the condition in which all of the features of API, VR/Aux.Ctrl., LPRC and HC are included. Similar to night-time operation, for a3-phase 4-wire distribution system, the current unbalance compensationfeature is achievable during the day-time too.

The preceding embodiments disclose several control aspects of theinvention. The successful realization of the disclosed control aspectsdepend mostly on the amount of reactive power injected by the PV solarfarm inverter (except for load balancing in which certain amount ofactive power is exchanged between load, inverter and grid). During thenight-time mode of operation, a small amount of active power is drawn bythe solar farm inverter to operate in self-supporting mode. The maximumreactive power that can be supported by a PV solar farm inverter isdependent on the MVA rating of that inverter. In the following section,the possibilities of reactive power support by a PV solar farm inverterare mathematically represented.

During Night-Time:

$\begin{matrix} \begin{matrix}{{P_{SF} = 0},{therefore},{Q_{SF} = {Q_{{SF}\mspace{14mu}\max} = S_{{SF},{rated}}}}} \\{I_{SF} = I_{SFq}} \\{\varphi_{SF} = 90^{o}}\end{matrix} \} & (4)\end{matrix}$

During Day-Time:

For rated power generation (100%)

$\begin{matrix} \begin{matrix}{{P_{SF} = {P_{{SF}\mspace{14mu}\max} = S_{{SF},{rated}}}},{therefore},{Q_{SF} = 0}} \\{I_{SF} = I_{SFa}} \\{\varphi_{SF} = 0^{o}}\end{matrix} \} & (5)\end{matrix}$

For power generation less than the rated value (<100%)

$\begin{matrix} \begin{matrix}{S_{{SF},{rated}} = {P_{SF} + {j\; Q_{SF}}}} \\{{I_{SF} = {{\overset{->}{I}}_{{SF}\; a} + {\overset{->}{I}}_{SFq}}},} \\{\varphi_{SF} \neq 90^{o} \neq 0^{o}}\end{matrix} \} & (6)\end{matrix}$

From (5), when the power generation from PV solar farm is at its ratedvalue during day-time, the solar farm inverter cannot be used to providethe reactive power. For lesser active power generation, there is alwaysan opportunity to provide simultaneous active and reactive power.

FIG. 9 shows an active-reactive powers (P-Q) capability curve drawn onthe basis of rated PV solar farm inverter capacity. The x-axisrepresents the possible values of active powers and the y-axisrepresents the possible values of reactive powers that the PV solar farmcan support without an increase in available inverter rating. The P-Qdiagram is divided in four regions based on the phase angle (φ_(SF)) ofnet injected current I_(SF) (φ_(SF) is measured with respect to the PCCvoltage), namely, Region—I, II, III and IV.

Ideally, the PV solar farm inverter should not consume any activepower—there is therefore no activity in Region-I and Region-IV. However,using the present invention, especially during night-time, the PV solarfarm will draw a very small amount of active power to maintain thevoltage across the DC side capacitor. This active power is essential toovercome the losses associated with the inverter. When the PV solar farmdoes not produce any active power, the available reactive power capacityis 100%. As can be seen from FIG. 9, when the PV solar farm generatesonly 20% of rated power (early morning/evening hours), up to 97.9%reactive power is available for different compensations. Interestingly,95% power generation still provides 31% of reactive power capacity thatcan be gainfully utilized.

In another preferred embodiment of the invention, an improved solar farminverter is provided to support reactive power while injecting maximumrated power. To achieve reactive support while injecting maximum ratedpower, the solar farm inverter is provided with an increased power (MVA)rating. In a preferred embodiment, even a moderate over-sizing of thesolar farm inverter provides significant benefits. In one example, if asolar farm inverter is over-sized by 5% to 10%, the available reactivepower capacity left to perform other tasks would be 32% to 45.8% using100% active power injection capacity.

The significant benefits provided by the above embodiment can beunderstood in an example in which a utility company needs to install aSTATCOM to regulate the PCC voltage. In this case, if utility wants toprovide 100% reactive power capacity, the required STATCOM rating wouldalso be 100%.

From the above, one preferred embodiment of the invention, shows thatsimply over-rating the PV solar farm by 41.2% would provide the samecapability as a separately installed 100% capacity STATCOM. Furthermore,one additional benefit with this over-sized (141%) inverter is that,during night-time when there is no active power generation, the reactivepower capacity of inverter also would increase from 100% to 141%.

The STATCOM is rated based on its apparent power rating which isdirectly dependent on its semiconductor switches' voltage and currentrating. The general manner of expressing the rating/capacity ofelectrical power related to electrical devices is by defining its MVA(Mega volt ampere; M for Mega, V for voltage, A for current in ampere).

FIG. 10 shows an exemplary block diagram representation of the controlscheme used to achieve the preferred control concepts in which the solarfarm is adapted to perform as a STATCOM and/or shunt active powerfilter. The exemplary control scheme is applicable both during the nightand day times. The controller has six different loops, namely (a)synchronization, (b) PCC voltage regulation and damping control, (c) DCbus voltage regulation, (d) load current harmonic compensation, (e) loadreactive power compensation and (f) active power injection.

A phase locked loop (PLL) is used to maintain synchronization with PCCvoltage. The PLL gives output in terms of sine and cosine functions. Thecosine functions are used to generate the reference quadraturecomponents of currents to regulate PCC voltage. The sine functions areused to generate the in-phase reference current components. Thesecomponents draw necessary fundamental active power to maintain the DCbus voltage at a predefined reference value. PCC and DC bus voltagecontrol loops are composed of proportional-integral (PI) controllers.

In a preferred embodiment of the invention, an auxiliary controller isadded in the PCC voltage regulation loop. This auxiliary controller canprovide stabilization and damping controls for several proposedapplications of the solar farm. Both the structure and operation of theauxiliary controller have already been described above.

To regulate the PCC voltage, the actual voltage at PCC is sensed andcompared with a reference value V^(*) _(pcc) of 1 pu. The output of theauxiliary controller is added to the voltage reference. The differencebetween the actual and reference voltages and auxiliary signal is thenprocessed with the Proportional Integral (PI) regulator. The output ofPI regulator is amplified with gain (k_(VR)) to generate the referencecurrent magnitude (I_(VR)). The current magnitude I_(VR) is thenmultiplied with cosine functions (‘cos a’, ‘cos b’ and ‘cos c’) togenerate the reference quadrature components (i_(VR,abc)) which willregulate the PCC voltage. Similarly, the reference signals i_(DC,abc)required to maintain the DC bus voltage constant are generated usingsine functions, especially during night-time. The signal V_(Er,cmd) inPCC voltage regulation loop is extracted for use in the master controlunit. This activates/deactivates the voltage regulation loop.

Generally, in the real-time implementation, the control scheme isdeveloped using a sophisticated digital controller (such as amicrocontroller, digital signal processor [DSP], etc.). All thenecessary quantities required in the control approach, (e.g. in ourcase, different voltages and currents) are sensed using voltage andcurrent sensors (such as Hall-effect transducers). These sensors,regardless of whether they are used to determine voltage or current orany other parameter in real-time, provide an output which is a “scaledvoltage signal”. For example, to sense a 120 kV voltage, the sensor mayhave an output of 1 volt as a representative signal. The user hascontrol over the setting of the sensor gain which can adjust the outputvalue. A similar situation exists for current measurement in that theuser has control over sensor gain and, as such, can adjust the outputvalue. These scaled signals are then converted into digital signals byusing an analog to digital converter. The user then multiplies thenecessary gain in DSP to extract the exact value of the sensed signal.For example, a 1 volt signal can be multiplied by 120,000 to obtain theexact value of the sensed signal. These gains are constant values and donot need to change or be affected by any variation in the sensedsignals. In the present invention, reference currents are beinggenerated which will be injected through the PV solar farm inverter toachieve different control aspects. For ease of understanding, it shouldbe noted that the signal corresponding to voltage is denoted as‘voltage’ and the signal corresponding to current is denoted as‘current’. As mentioned above, all these signals in DSP are ‘voltages’.Since the mathematical computations/operations in executed in DSP, theterms ‘voltage’/‘current’/‘power’ etc. do not have significant meaningas they are all representative signals.

DC bus voltage regulation mode is applied only during the night-timemode of operation to provide a self-supporting DC bus across the PVsolar farm inverter. The DC bus capacitor is usually charged from theelectrical output of the solar panels. During night time, since there isno solar power produced, this DC bus capacitor still needs to be keptcharged to supply the reactive power expected by the STATCOM operation.The solar arrays should be isolated from the DC bus capacitor bydisconnecting them through mechanical switches. This helps to ensurethat the solar arrays will not be damaged due to sudden surges involtage/current.

The DC bus voltage control loop is also comprised of aproportional-integral (PI) regulator. To regulate the DC voltage, theactual DC bus voltage is sensed and compared with an appropriatelyselected reference value V^(*) _(dc). The difference between the actualand reference voltages is then processed with the PI regulator. Theoutput of the PI regulator is amplified with a proper gain (k_(v)) togenerate the reference current magnitude I_(DC). The current magnitudeI_(DC) is then multiplied with sine functions (‘sin a’, ‘sin b’ and ‘sinc’) to generate the in-phase reference components (i_(dc,abc)). Thesecomponents draw the necessary fundamental current component (activepower) to maintain the DC bus voltage at the reference level. Thisactive power is needed to overcome the losses associated with theinverter and passive elements (e.g. coupling inductance, DC buscapacitor, etc.) during STATCOM operation.

To provide the load reactive power and to compensate for currentharmonics (if any), the instantaneous determination of different activeand reactive powers is used—the active and reactive powers are computedusing single phase p-q theory. This approach is used as it allowsseparate or combined load reactive and current harmonic compensations.Additionally, in case of unbalanced load condition, an easy expansion toinclude load balancing is possible. Using the concept of single-phasep-q theory, a three-phase system is represented as three separatesingle-phase systems and the single-phase p-q theory is applied to eachphase independently.

Considering phase-a, the PCC voltage and the load current can berepresented in α-β coordinates as:

$\begin{matrix}{\begin{bmatrix}v_{{PCC},{a{\_\alpha}}} \\v_{{PCC},{a{\_\beta}}}\end{bmatrix} = \begin{bmatrix}{v_{{PCC},a}( {\omega\; t} )} \\{v_{{PCC},a}( {{\omega\; t} + {\pi/2}} )}\end{bmatrix}} & (7) \\{\begin{bmatrix}i_{L,{a{\_\alpha}}} \\i_{L,{a{\_\beta}}}\end{bmatrix} = \begin{bmatrix}{i_{L,a}( {{\omega\; t} + \varphi_{L}} )} \\{i_{L,a}\lbrack {( {{\omega\; t} + \varphi_{L}} ) + {\pi/2}} \rbrack}\end{bmatrix}} & (8)\end{matrix}$

Using the concept of single-phase p-q theory, the instantaneous activeand reactive powers are determined as:

$\begin{matrix}{\begin{bmatrix}p_{La} \\q_{La}\end{bmatrix} = {\begin{bmatrix}v_{{PCC},{a{\_\alpha}}} & v_{{PCC},{a{\_\beta}}} \\{- v_{{PCC},{a{\_\beta}}}} & v_{{PCC},{a{\_\alpha}}}\end{bmatrix} \cdot \begin{bmatrix}i_{L,{a{\_\alpha}}} \\i_{L,{a{\_\beta}}}\end{bmatrix}}} & (9)\end{matrix}$

Total instantaneous active (p_(La)) and total instantaneous reactivepower (q_(La)) can be decomposed into fundamental and harmonic powersas:p _(La) =p _(La) +{tilde over (p)} _(La)  (10)q _(La) =q _(La) +{tilde over (q)} _(La)  (11)

In (10) & (11), p _(La) and q _(La) represent the DC components, whichare responsible for fundamental load active and reactive powers. {tildeover (p)}_(La) and {tilde over (q)}_(La) represent the AC componentswhich are responsible for harmonic powers. The fundamental instantaneousload active (p _(La)) component and the fundamental instantaneous loadreactive (q _(La)) component can be extracted easily from p_(La), andq_(La), respectively, by using a low pass filter (LPF). Furthermore, theinstantaneous harmonics active ({tilde over (p)}_(La)) and reactivepower ({tilde over (q)}_(La)) components can be separated from the totalpower by using a high pass filter (HPF). Thus, using the concept ofsingle-phase p-q theory, different active and reactive powers can becalculated separately in real-time.

For load current harmonic compensation, the solar farm inverter shouldsupply the harmonic part of the load current. That is, the referencecurrent signal generation should be based on terms {tilde over (p)}_(La)and {tilde over (q)}_(La).

Therefore for phase-a,

$\begin{matrix}{{\begin{bmatrix}i_{{HC}{\_\alpha}} \\i_{{HC}{\_\beta}}\end{bmatrix} = \frac{1}{A_{xa}}}{\cdot \begin{bmatrix}v_{{PCC},{a{\_\alpha}}} & v_{{PCC},{a{\_\beta}}} \\v_{{PCC},{a{\_\beta}}} & {- v_{{PCC},{a{\_\alpha}}}}\end{bmatrix} \cdot \begin{bmatrix}{\overset{\sim}{p}}_{La} \\{\overset{\sim}{q}}_{La}\end{bmatrix}}{{where},}} & (12) \\{A_{xa} = {v_{{PCC},{a{\_\alpha}}}^{2} + v_{{PCC},{a{\_\beta}}}^{2}}} & (13)\end{matrix}$

Since α-axis quantities represent the original system, the referencecurrent for load current harmonic compensation can be given as:

$\begin{matrix} {{i_{{HC},a}( {\omega\; t} )} = {\frac{1}{A_{xa}} \cdot \lbrack {{{v_{{PCC},{a{\_\alpha}}}( {\omega\; t} )} \cdot {{\overset{\sim}{p}}_{L,a}( {\omega\; t} )}} + {{v_{{PCC},{a{\_\beta}}}( {\omega\; t} )} \cdot {{\overset{\sim}{q}}_{L,a}( {\omega\; t} )}}} \rbrack}} \rbrack & (14)\end{matrix}$

Similarly, the reference current for load current harmonic compensationfor phase-b and phase-c are also estimated.

For fundamental load reactive power compensation, the reference currentshould be based on only the term q _(La).

Therefore for phase-a,

$\begin{matrix}{{\begin{bmatrix}i_{{LRPC}{\_\alpha}} \\i_{{LRPC}{\_\beta}}\end{bmatrix} = \frac{1}{A_{xa}}}{\cdot \begin{bmatrix}v_{{PCC},{a{\_\alpha}}} & v_{{PCC},{a{\_\beta}}} \\v_{{PCC},{a{\_\beta}}} & {- v_{{PCC},{a{\_\alpha}}}}\end{bmatrix} \cdot \begin{bmatrix}0 \\{\overset{\_}{q}}_{La}\end{bmatrix}}} & (15)\end{matrix}$

The reference current for load reactive power compensation can be givenas:

$\begin{matrix}{{i_{{LRPC},a}( {\omega\; t} )} = {\frac{1}{A_{xa}} \cdot \lbrack {{v_{{PCC},\beta}( {\omega\; t} )} \cdot {{\overset{\_}{q}}_{L,a}( {\omega\; t} )}} \rbrack}} & (16)\end{matrix}$

Similarly, the reference current for load reactive power compensationfor phase-b and phase-c are also estimated.

The active power generated from the PV solar plant is transferred to themain grid through a proper controller, for example, in the maximum powerpoint tracking (MPPT) mode. Finally, all the control loop currentcomponents are added together to generate the overall reference currentsignals (i*_(SF,abc)) for the solar farm inverter. These referencesignals are then compared with actual sensed solar farm inverter outputcurrents (i_(SF,abc)) and processed using a hysteresis currentcontroller to perform switching of inverter semiconductor devices.

FIG. 11 depicts the block diagram of a Hysteresis current controller. AHysteresis controller gives a switching instant (for example, G1)whenever the error exceeds a fixed magnitude limit i.e. a hysteresisband. In order to avoid a short circuit, an opposite signal is appliedto switch S6. A “NOT” gate is used to generate the desired S6 pulse. Byusing three hysteresis controllers, one for each phase, the gatingsignal pattern (G1 to G6, see FIG. 2) for the PC solar farm inverter isgenerated.

All the reference signals for different functionalities are generated ona continuous basis and the master control unit is used toactivate/deactivate different loops based on priorities and controlrequirements. For example, the voltage regulation mode is activated onlyif the PCC voltage rises/drops below the set reference value of ±1%(1.01 pu or 0.99 pu). The current harmonic compensation loop isactivated if the THD in load current is noticed to be more than 5%.

An exemplary flow chart for the master control unit is given in FIG. 12.A priority is assigned to each of the tasks. The primary use of solarfarm inverter is for injecting the available PV solar power to the gridduring the day-time. Therefore, the active power injection loop has beengiven the highest priority. Since it is important to have aself-supporting DC bus so as to achieve different tasks duringnight-time, this task has been given the second highest priority. Itshould be noted that care must be taken not to activate both the loopssimultaneously. Similarly, other loops have been assigned hierarchicalpriorities. The master control unit generates five priority basedcontrol commands, namely, u′_(API), u′_(VDC), u′_(VR), u′_(HC) andu′_(LRPC). These control commands can have “0” or “1” value and aremultiplied with respective control loop reference current components toactive or deactivate it.

The inverter controller, shown schematically in FIG. 11, may beimplemented using several different types of semiconductor deviceswitches such as GTOs, IGBTs, IGCTs, etc. For example, those skilled inthe art would readily appreciate that the present invention is equallyapplicable for single-phase and three-phase four wire systems. Thepresent invention is also applicable to a three-phase three-wire system.

The present invention is typically more beneficial for a large-scale DGsystem. To regulate the feeder voltage when the system voltage is high(e.g. 12.7 kV, 27.6 kV, etc.), the PV solar farm capacity should be highenough (i.e. in the order of megawatts) to give satisfactory results.The present invention is equally applicable to smaller size DG systemswith the caveat that such implementations would have reduced networkcompensation capability.

The present invention is also applicable for small capacity PV solarfarms. However, as mentioned earlier, the compensation capability isdependent on the sum of individual PV solar farm inverter ratings. Ifthere are many small PV solar farms in close vicinity, using a morecomplex control approach, all the small PV solar farms can be seen asone large unit. By dividing the control objective into parts, the sameperformance as that of using a single high rated PV solar farm can beachieved. For example, if a 1 MW solar farm can control the PCC voltageas a STATCOM by injecting 1 MVAR reactive power, then, 10 PV solarfarms, each of 100 kW capacity (connected close to each other), canperform the same operation by supporting 100 kVAR reactive power fromeach of 10 PV solar farm inverter.

All the proposed embodiments and capabilities of the invention can beachieved for any type of distribution network, be it of radial type ormeshed type.

While the preceding embodiment of the invention provided a system andmethod for adding additional wind farms to a DG network by adapting asolar farm inverter to operate as a STATCOM, the invention is notlimited to wind farms as existing or additional DG systems. Any otherinverter based DG system that is inactive at any point of time (day ornight) for any reason, can also be utilized as a STATCOM as describedabove. Such a DG system could be a large inverter based wind farm or aFuel Cell based DG. The present invention provides for the utilizationof an inactive inverter which may come from any DG at any time.

It is important to note that the system shown in FIG. 10 is merely anexample of the components required to achieve the operation of a solarfarm as a STATCOM and shunt active power filter, and those skilled inthe art will readily understand that the present invention furthercontemplates other related methods and systems. For example, theinverter may be switched with switching means other than a hysteresiscurrent controller, such as other power semiconductor switching devicesknown in the art that include, but are not limited to, GTOs, IGBTs,IGCTs, etc.

Furthermore, while the processing elements shown in

FIG. 11 are shown as discrete elements, they may be provided in a singledevice, such as a computer processor, an ASIC, an FPGA, or a DSP card.

In a further embodiment, the present invention provides a voltagecontrol and a damping control with a grid connected inverter based solarDG, or an inverter based wind DG, to improve the transient stability ofthe system whenever there is an availability of reactive power capacityin the DGs. This aspect of the present invention has been studied andperformed for two variants of a Single Machine Infinite Bus (SMIB)system. One SMIB system uses only a single solar DG connected at themidpoint whereas the other system uses a solar DG and a converter basedwind DG. Three phase fault studies are conducted using theelectromagnetic transient software EMTDC/PSCAD, and improvements instable power transmission limit are investigated for differentcombinations of controllers on the solar and wind DGs, both during nightand day.

The single line diagrams of two study systems—Study System 1 and StudySystem 2 are depicted in FIG. 13 (a) and FIG. 13 (b), respectively. Bothsystems are Single Machine Infinite Bus (SMIB) systems in which a largesynchronous generator (1110 MVA) supplies power over a 200 km, 400 kVtransmission line to the infinite bus.

In Study System 1, a single inverter based Distributed Generator (asolar farm in this case) is connected at the midpoint of thetransmission line. In Study System 2, two inverter based DGs areconnected at ⅓rd and ⅔rd line length from the synchronous generator. TheDG connected at ⅓rd distance is considered to be a wind farm utilizingPermanent Magnet Synchronous Generators (PMSG) with ac-dc-ac converters,whereas the DG connected at ⅔rd distance is considered to be a solarfarm. It is understood that both the solar farm and wind farm will haveseveral inverters in each of them. However, for this analysis, each DGis represented by a single equivalent inverter having a total rating ofeither the solar farm or wind farm. Both the wind farm and solar farmare considered to be of the same rating, and therefore can beinterchanged in terms of location depending upon the studies beingperformed. FIG. 14 illustrates the block diagrams of the varioussubsystems in the two equivalent DGs.

The synchronous generator is represented in detail by a sixth ordermodel and a DC1A type exciter. The different transmission line segmentsTL1, TL2, TL11, TL12, TL22, shown in FIG. 13 are represented bycorresponding lumped pi-circuits. Saturation is neglected in both thesending end and receiving end transformers.

The solar farm and wind farm, as depicted in FIG. 14, are each modeledas equivalent voltage sourced inverters along with pure DC sources. Inthe solar farm, the DC source is provided by the solar panels output,whereas in the wind farm, the PMSG wind turbines rectifier outputgenerates the DC voltage source. The DC power output of each DG is fedto the DC bus of the corresponding inverter to inject real power to thegrid, as illustrated in FIG. 14 (a). The magnitude of real powerinjection from the DGs to the grid depends upon the magnitude of DCinput voltage. The voltage source inverter in each DG is composed of sixIGBTs in a matrix with snubber circuits as shown by ‘IGBT matrix’ blockin FIG. 14 (a). A large size DC capacitor is used to reduce the DC sideripple. Each phase has a pair of IGBT devices which convert DC voltageinto a series of variable width pulsating voltages according to theswitching signal to the matrix utilizing the sinusoidal pulse widthmodulation (SPWM) technique. Switching signals are generated from theamplitude comparison of variable magnitude sinusoidal signal known as‘modulating signal’ with high frequency fixed-magnitude triangularsignal known as ‘carrier signal’ as shown in the ‘gate pulse generation’block in FIG. 14. The variable magnitude and the phase angle ofsinusoidal modulating signals are controlled by either one of theexternal controllers—‘control system I’ block in FIG. 14 (a) or ‘controlsystem II’ block in FIG. 14 (b), which modifies the switching signalwidth duration. The modulating signals used for three phases are equallyspaced and thereby shifted by 120° whereas the same carrier wave is usedfor all three phases. Some filter equipment may be needed at the AC sideto eliminate harmonics. In this model the carrier signal amplitude isnormalized to unity, hence the magnitude of modulating signal isalternately designated as modulation index (MI).

In the PWM switching technique, the magnitude of voltages and the angleof voltages at the inverter output are directly dependent on themodulation index (MI) and on the modulation phase angle, respectively.To control the modulation index and the modulation phase angle, twoseparate PI control loops are simultaneously integrated with theinverter. The different DG control systems utilized are described below.

i) Control System 1: This contains two Proportional Integral (PI)controllers, as depicted in FIG. 14 (a). The lower PI controller is usedto maintain the voltage, VDC, across the DC link capacitor, whereas theupper PI controller, known as the reactive power controller, is utilizedto directly control the flow of reactive power from the DG to the PCCthrough the control of the modulation index. The measured reactive powerflow from the DG is therefore used as controller input and compared withQref. Normally, the DGs are required to operate at almost unity powerfactor and therefore in the conventional reactive power control of theDGs, the Qref is set to zero.

ii) Control System II: This control system also comprises two PIcontrollers as shown in FIG. 14 (b). The upper PI controller, known asvoltage controller is mainly used to regulate the PCC voltage to apredefined set point. This controller regulates the PCC voltage throughthe control of modulation index and thereby uses the PCC voltage ascontroller input. As the amount of reactive power flow from the DGinverter depends upon the difference in magnitudes of voltages at PCCand inverter terminal, the DG reactive power flow can also be controlledindirectly with this control system. In this control system also, thelower PI controller is used to maintain the voltage, VDC, across the DClink capacitor.

iii) Damping controller: A novel auxiliary ‘damping controller’ shown inFIG. 14 (a) is utilized to damp the rotor mode (low frequency)oscillations of the synchronous generator and to thereby improve thesystem transient stability. This damping controller is appended to bothControl System 1 and Control System 2. In this controller, the linecurrent magnitude signal is utilized as the control signal which sensesthe rotor mode oscillations of the generator. The magnitude of linecurrent signal is passed through a washout function in series with afirst order lead lag compensator.

The damping controller can be used as a supplementary controllertogether with either the voltage controller or reactive powercontroller. The parameters of the reactive controller, the voltagecontroller and auxiliary controller are tuned by a systematic hit andtrial method, in order to give the fastest step response, least settlingtime and a maximum overshoot of 5%.

In summary, the present invention provides numerous novel embodimentsinvolving the use of a solar farm as a STATCOM in a distributed powergeneration network and additional functions through controlled reactivepower injection, and in particular:

-   -   The Solar farm can be utilized as a STATCOM for grid voltage        control allowing the integration of an increased number of wind        turbine generators and other renewable/non-renewable distributed        generators in the transmission/distribution line.    -   The solar farm can be operated as a STATCOM to increase the        power transmission capacity of transmission lines to which they        are connected. Increasing transmission capacity is a great        challenge faced by electric power utilities around the globe. PV        Solar farms can play that role both during nights as well as        during the days.    -   The solar farm can be operated as a STATCOM to improve the        system stability thereby helping prevent blackout scenarios.    -   The solar farm can be operated as a STATCOM to enhance the        damping of low frequency (0.2-2 Hz) power oscillations thus        helping increase the power flows in transmission systems. This        problem exists in several countries around the world.    -   Synchronous generators that are connected to series compensated        transmission lines to increase the power transmission capacity,        but are subjected to the problem of sub-synchronous resonance        (SSR) that if uncontrolled, can result in enormously expensive        generator shaft failures/breakages. If a solar farm is located        close to synchronous generator, it can be operated as a STATCOM        to mitigate sub-synchronous resonance.    -   Alleviation of voltage instability: systems having large        reactive power consuming loads such as induction motor loads,        steel rolling mills, etc, are subject to the problem of voltage        instability (sudden reduction/collapse of the bus voltage) under        line outages, or faults. Solar farms in the vicinity of such        loads can be operated as a STATCOM to provide very rapid voltage        support to mitigate this problem of voltage collapse.    -   Limiting short circuit currents: transmission and distribution        networks are facing a huge problem of high short circuit        currents as new renewable/non-renewable energy sources are being        connected to the grid, as each source contributes to current in        the faulted network. The solar farms inverter can be operated in        an entirely novel manner to operate as a rectifier during the        short circuits to thereby suck the fault current back from the        fault and charging its own capacitor. In this manner the PV        solar farms will allow more connections of new generating        sources in the grid.    -   Improvement of High Voltage Direct Current (HVDC) converter        terminal performance: solar farms near HVDC lines can provide        dynamic voltage support to successfully operate the HVDC        converters even under very stringent (weak) network conditions    -   Solar farms as STATCOMs can provide the low voltage ride through        (LVRT) capability for successfully integrating wind farms.        During faults the line voltage reduces to very low values        causing the nearby wind farms to get disconnected. Solar farms        can provide voltage support during these situations to allow the        wind farms to remain connected and continue to supply power to        the grid.    -   The PV solar farm can act as an Active Power Filter to perform        power factor correction, balancing of unsymmetrical loads and        line current harmonic compensation, all in coordination with the        abovementioned functions of FACTS.    -   All of the above objectives can be achieved during the day-time        also by solar farms.    -   If the PV solar farms are provided with energy storage        capability in the form of storage batteries, the solar farm can        be utilized as a battery charger during night-times when there        is excess power production by neighbouring wind farms and the        loads are much less. This stored power can be sold to the grid        during day-time when needed by the grid at very attractive        prices.    -   Such energy storage will also help shave the peak power demand        in electrical networks. During peak hours, instead of the grid        importing power at high rates, it can buy stored power from        solar farms to meet the peak demands. This application will be        in limited situations when the solar farm is not producing its        peak/rated power, but still be very valuable.

In addition to the above, there are many other advantages to utilizing avoltage control and a damping control on an inverter-based DG (both PVsolar and wind) for improving the transient stability and, consequently,the power transmission limit in transmission systems. A number of thesereasons are:

-   -   The solar DG, which is presently not at all utilized at night        times, can now be utilized with the proposed voltage and damping        control to increase the power transmission limit significantly        at night-times. Even during day-time when the solar DG produces        a large magnitude of real power, the controllers can help        increase the stable transmission limit to a substantial degree.        The choice of the voltage reference in the voltage controller        must be made judiciously to get the maximum improvement in power        transfer. For the study system I, a 100 MW solar farm can        increase transmission limit by about 200 MW in the night and by        97 MW during the daytime.    -   When both solar and wind DGs, of 100 MW each, are connected to        the system operating with the damping control, the transmission        capacity is seen to increase by 240 MW if no DGs are producing        real power output, and by 141 MW if both are producing a high        level of real power of 94 MW.    -   When both solar and wind DGs are connected to the system,        operating with the damping control, and only one DG is producing        real power, the power transfer limit increases even further by        at least 356 MW.    -   The DG FACTS devices of the present invention improve the        transient stability and, consequently, the power transfer limit        of the grid. These can also be used to provide other        functionalities of the FACTS devices.    -   Embodiments of the present invention are fully extendable to        other inverter-based DGs, such as Doubly Fed Induction Generator        (DFIG) based wind turbine generators.

The solar farm DG can generate further revenue for its operators bybeing operated as a STATCOM. As noted above, the STATCOM-operated solarfarm can increase the transmission capacity of power transmissionsystems. By charging a suitable fee to the operators of wind farm DGscoupled to the transmission system or to the operators of utilitycompanies for increases in the transmission capacity of the transmissionsystem, operators of the solar farm DG can share in the financialbenefits of the increased transmission capacity. This method wouldentail operating the solar farm DG as a STATCOM at night or whenever thesolar farm inverter is not being fully utilized in real power generationand charging utilities or the other energy farm operators for thebenefit of increased transmission capacity. Of course, the charges couldbe based on a percentage of increase in the transmission capacity, onthe amount of time the solar farm DG is being used to the benefit of theother energy farm DGs, or any other combination of factors.

It should be noted that the method outlined above regarding the use of asolar energy farm to increase the transmission capacity of transmissionlines may also be used on wind energy farms.

Further revenue can be generated by solar energy farms by chargingutility companies or other interested parties for using the solar energyfarms for transmission and distribution grid voltage control. As notedabove, inverter equipped solar energy farms, when operated as STATCOM,provides voltage control for the power transmission grid and allows formore wind farms to be coupled to the same grid to which the solar farmsare coupled. By providing for more wind energy farms to be connected tothe transmission grid without having to invest in dedicated voltageregulating equipment, wind energy farm operators as well as powerutility companies save on capital expenditures. As such, solar farmenergy operators can charge either on-going fees to the wind farmoperators/utilities or a flat rate fee for the benefit provided by theirinverters used as STATCOMs.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiments encompassed within the following claims and theirequivalents.

What is claimed is:
 1. A system for improving stability of a power gridsystem, said system comprising: a power generating source having anoutput being injected on to said power grid system; a damping controllerreceiving as input a signal indicative of oscillations in said powergrid system, and said damping controller outputting a damping controlsignal; and a control system, being co-located with a voltage inverterof said power generating source, for receiving said damping controlsignal, wherein said control system outputs a control signalproportional to a transient signal on said power grid system; whereinsaid control signal controls said power generating source such that saidoutput is based on said control signal; wherein said control systemdirectly and independently operates said voltage inverter of said powergenerating source as a Static Synchronous Compensator (STATCOM) toexchange controlled reactive power to improve said stability of saidpower grid system; wherein said control system only operates saidvoltage inverter; said voltage inverter being operated as a STATCOM onlywhen said voltage inverter is operating at a capacity less than a ratedcapacity of said voltage inverter; and said control system operatingsaid voltage inverter as a STATCOM using an unutilized capacity of saidvoltage inverter, and wherein said damping controller comprises acompensator for compensating for said oscillations of said power gridsystem.
 2. The system according to claim 1, wherein said powergenerating source is a solar farm DG power source.
 3. The systemaccording to claim 1, wherein said power generating source is a windfarm DG power source.